A spring-slider model with a laboratory-derived constitutive law for serpentinite generates seismic slip, aseismic slip, or a combination of both. Such behavior is observed on oceanic ridge-transform systems and portions of the San Andreas Fault, and may be due to the presence of serpentinite. The constitutive law combines two sets of equations: a rate-dependent flow equation which dominates at low velocities, and a rate- and state-dependent set of equations which dominate at high velocities. The combined model produces transient behavior with maximum slip velocities well in excess of the loading velocity. Metastable slip prior to the onset of high-speed slip is produced in many simulations and may be analogous to the slow precursors observed for some oceanic ridge-transform earthquakes. This precursory slip results from the double-valued nature of the steady-state curve of the two-mechanism model in which the state-variable equations are rate weakening.